As mentioned in Patent Documents 1 to 3 for example, common photosensitive resin plates for flexography are produced via exposure, development and post-exposure steps. A printing method using the photosensitive resin plate for flexography is such a method wherein ink is supplied onto the convex surface of uneven resin plate using an ink-supplying roll or the like and then the printing plate is contacted with the material to be printed so that the ink on the convex surface is transferred to the material to be printed. In the flexography as such, adhesion of the ink to a shoulder area of the convex surface of the printing plate or invasion of the ink to the concave surface (hereinafter, it will be referred to “plate surface contamination”) often happens during the printing for long time. As a result, even the area which is not the inherent pattern area may be printed resulting in deterioration of the printing quality.
Particularly in recent years, application of flexography to a highly precise printing has been in progress and a halftone dot printing in high line numbers has been briskly carried out for creating the gradation of color and concentration. Since intervals among the halftone dots become smaller in that case, the ink is more apt to be filled in the concaves of the halftone dots of the printing plate. When the ink is accumulated in the concaves of the printing plate in a predetermined amount or more, it transfers to the material to be printed to generate the dot bridging resulting in a significant deterioration of the printing quality.
One of the characteristics of flexography is that it can cope with various types of ink such as aqueous ink, alcoholic ink, solvent-type ink or UV ink. On the other hand, there has been a demand for preventing plate surface contamination even when any type of ink is used. It is particularly necessary to prevent plate surface contamination even if an ink having high permeability to the plate material such as solvent-type ink or UV ink is used.
Several methods have been proposed already concerning the prevention of plate surface contamination of a flexographic printing plate. For example, Patent Document 4 discloses a method wherein silicone oil, silicone rubber, silicone varnish or fluorine compound is adhered, by means of brushing or spraying, onto a printing plate mainly comprising polyester resin. Although some preventing effects for plate surface contamination are achieved by such a method, the plate surface contamination cannot be prevented in a halftone dot printing in high line numbers having narrow intervals among the dots.
In Patent Document 5, there is proposed a method wherein a mixture of an aqueous resin and an aqueous emulsion of silicone compound or fluorine compound is applied onto a printing plate comprising an elastomer. In this method, due to the use of a non-modified silicone compound, an effect of preventing the plate surface contamination is not sufficient.
In Patent Document 6, there is proposed a method for contacting a flexographic printing plate with a solution containing a modified silicone compound after the exposure step during production of flexographic printing plate. In this method, an improvement in the effect of preventing the plate surface contamination is noted due to the use of a modified silicone compound. However, in a long-run printing, plate surface contamination generates and, its persisting effect is not sufficient.
In Patent Document 7, there is proposed a method wherein a liquid containing an amino-modified silicone compound is contacted with a flexographic printing plate containing latex having gelling degree of 50% by mass or more. In this method, an improvement in an ability to prevent plate surface contamination can be expected due to an interaction of the amino-modified silicone compound with the gelled latex and/or due to an interaction of the amino-modified silicone compound with an emulsifier. However, the method does not satisfy any of the prevention of plate surface contamination and persistency thereof whereby it is not sufficient for a high-quality printing in a large scale.
In Patent Document 8, there is proposed a method wherein an organosilicon compound is contained in a flexographic printing plate. In this method, an ink-repelling ingredient is contained in a resin composition and, as compared with a method wherein an ink-repelling ingredient is contacted with a plate surface, a better persisting effect is achieved. However, content of the organosilicon compound in the resin composition for the printing plate is as small as 0.05% by mass to 1.0% by mass and amount of the ink-repelling ingredient existing on the surface of the printing plate is small. Therefore, the ability to prevent plate surface contamination cannot be said to be satisfactory.